Author Topic: DC Load  (Read 2744 times)

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Offline peteroakes

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Re: DC Load
« Reply #25 on: December 01, 2017, 06:15:49 am »
I had no idea I was being talked about over here  8)
Glad the project is useful to you, I have another collaboration right now on Element14  for a full software controlled load, PCB designs and everything, you and others of Daves fans may find interesting
https://www.element14.com/community/docs/DOC-88209/l/programmable-electronic-load-adc-and-dac-boosterpack-test

 
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Offline paulca

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Re: DC Load
« Reply #26 on: December 01, 2017, 10:17:56 pm »
DAC/ADC control and metering is on my radar, for further down the line, thanks Peter.

On a different note:

Would this work or be naive for input protection?
5A Self-reseting fuse
https://www.bitsbox.co.uk/index.php?main_page=product_info&cPath=214_215_222&products_id=1534

I am more interest in protecting the power source than the dummy load in this case.  Say for example I am draining a LiPo battery and my mosfet fails to short circuit.  I would like, if at all possible to avoid the resultant LiPo explosion.  That might suck somewhat.

If not that, a traditional 5A plug top fuse or 5A cartridge fuse?

----------------

On a different topic any basic pointers in the direction of thermal limit circuitry?  I gather for a hard limiter a thermistor used in a voltage divider feeding a comparator can result in a high output up until the temperature hits a limit and then the comparator drops to a low output.  The question would be what to do with that high/low output.  If I put it to the base of a NPN 3904 in between the drive op amp and the mosfet would it

a) be able to cut the control signal to the mosfet to below 2V ( I expect so )
b) interfere enough with the drive feedback to alter the performance of the device

Also, if I removed the comparitor and drove the 3904 with the termistor voltage divider could it be made to soft limit, ie increasingly backing out the mosfet gate voltage above a certain temperature.

Would applying a voltage from the low side supply rail into the feedback of the op amp via the 3904 work?  So if the termistor divider rises over 0.6V the 3904 will start feeding extra voltage into the sense feedback line backing the opamp and thus mosfet off.
« Last Edit: December 01, 2017, 10:22:24 pm by paulca »
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Current Open Projects:  3 Channel Audio mixer with DAC, BT, pre-amps and h/phone amp, WS281x LED controller Version 2 5V/5A w/Atmega328, FY6600 Power supply, 5A DC Load (Still!)
 

Offline paulca

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Re: DC Load
« Reply #27 on: December 01, 2017, 10:53:21 pm »
Would this work or be naive for input protection?
5A Self-reseting fuse

http://www.mouser.com/ds/2/240/Littelfuse_PTC_Rline_Catalog_Datasheet.pdf-1021735.pdf

So if I am reading the datasheet correctly this would not work for my needs.

A polyfuse rated for 5A hold would not actually trip until it gets to 10A or so and even then it could take 14 seconds to trip (at 25A!).  14 seconds at 25 amps, is probably fine for a LiPo, but it seems the device itself is only rated to 100A MAX, when I assume it detonates or otherwise fails.

They also have horrible temperature derates meaning the 5A hold fuse could actually begin to trip much lower if it gets hot near the Mosfet (or just by the current flowing through it).  So the dummy load could cut out as low as 4A after a prolonged period trips the poly fuse.

The poly fuse could be used as a much lower sustained current limiter, to allow bursts of higher current but I expect for the upper short circuit protection I need an actual fuse.
« Last Edit: December 01, 2017, 10:54:59 pm by paulca »
"What could possibly go wrong?"
Current Open Projects:  3 Channel Audio mixer with DAC, BT, pre-amps and h/phone amp, WS281x LED controller Version 2 5V/5A w/Atmega328, FY6600 Power supply, 5A DC Load (Still!)
 

Offline paulca

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Re: DC Load
« Reply #28 on: December 02, 2017, 01:43:44 am »
Is this an anomaly with the Falstad sim or am I having a blonde moment?

I found the idea from a thermistor product site, they had a circuit which pulled the mosfet gate low using a thermistor if it reached a thermal limit.

When I attempt to simulate it, it doesn't matter how hard I open the NPN transistor the gate stays high and open.

http://tinyurl.com/yajtt862

Here is the product circuit:

http://www.amwei.com/news.asp?news_id=89

What did work in Falstad was reversing the idea and adding voltage into the sense loop.  However for that to work I had to put a diode on the sense line, but falstad did not simulate the forward drop across the diode.  If the forward drop proportional to current flow?  There is obviously close to zero current flowing on that line.
"What could possibly go wrong?"
Current Open Projects:  3 Channel Audio mixer with DAC, BT, pre-amps and h/phone amp, WS281x LED controller Version 2 5V/5A w/Atmega328, FY6600 Power supply, 5A DC Load (Still!)
 

Offline PA4TIM

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Re: DC Load
« Reply #29 on: December 02, 2017, 02:22:48 am »
If the forward drop proportional to current flow?  There is obviously close to zero current flowing on that line.

Yes, the forward drop is totally depending on the forward current. It starts at zero and can go up to over 1V. See datasheet from your diode. That does not go lineair. The current from the diodetest in a DMM will result somewhere between 0,6 and 0,8V depending on the type of diode and the current from your meter.
Also, a diode needs some current, so with no load and a very high resistance input  DMM (>1Gohm) you will measure some fantasy value because there is no current flow, and no current is no voltdrop.
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Offline paulca

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Re: DC Load
« Reply #30 on: December 02, 2017, 02:31:59 am »
Yes, the forward drop is totally depending on the forward current.

So in theory this should work without altering the sense voltage?

http://tinyurl.com/ya37wn7l

EDIT:  I'm baffled, this shouldn't work.  The 100mV sense voltage doesn't come close to the 800mV forward voltage on the diode.
« Last Edit: December 02, 2017, 03:05:16 am by paulca »
"What could possibly go wrong?"
Current Open Projects:  3 Channel Audio mixer with DAC, BT, pre-amps and h/phone amp, WS281x LED controller Version 2 5V/5A w/Atmega328, FY6600 Power supply, 5A DC Load (Still!)
 

Offline paulca

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Re: DC Load
« Reply #31 on: December 03, 2017, 03:13:51 am »
Anyway, phase two was a success.  Using the big mosfet and the CPU heatsink.

Gotta love AMD OEM Coolers.  It's like a small block laptop cooler.  Using my "Lighter Putter Outer 100" from my other tongue in cheek post to run the heatsink fan and everything worked the first time I plugged a load into it.

I even trusted it enough to connect my biggest LiPo to it.  5Ah 25C 12.6V (capable of an alleged 125A).   SAFETY TIP!: Along with a LiPo analyser which gave me a loud alarm if the pack unbalanced or a cell dropped below 3V.

With the ExTech monitoring true current I trusted the Chinese battery analyser to watch the voltage and cell volts.  It's current reading disagreed with the Extech by 400mA, so I ignored it.

Very impressed by the AMD cooler.  With my kitchen temperature  probe stuck in the heat grease beside the mosfet the highest temp I recorded was 37.4*C with the load pulling 3.0A  It was able to consistently hold that temperature for half an hour at 3.0A while the lipo discharged and I could see the temp fall by about 0.1*C for each few 100mV the LiPo dropped by.  Even at 3 amps I could comfortably keep my finger on the mosfet case.

The little thermistor lighter putter outer worked perfectly.  The thermistor all insulated up and stuff into the heatsink core. When I put the load on the fan started slow after a few seconds, got to full speed fairly quickly, drawing 100mA.  When the load was shut off the fan continued to run for 10 or 20 seconds before shutting off.  When the load was running at only 0.5A the fan ran at about 50%.  I have no idea how I managed to randomly set the thing perfectly without trying, but it pretty much is perfectly tuned.  If anything the fan could be backed off a bit more.

I have days when things always fail and I get frustrated.  Today, everything worked.  Pretty much the first time I plugged it in and tried it, it worked.

There are bugs however.  The PNP running the fan is sinking 4-5mA for no good reason when it should be off and the fan shouldn't be that variably when it's being driven from a LM393 comparator, it should be a hard on/off.  I'm not sure I want to fix the latter, but I will probe the thing to see what's going on.  Also when I touch the breadboard or even the heatsink I get another 200mA current, so I have open noise being introduced from my hand somehow.  (I really should probe it!)

I couldn't test it to 5A as I'm still using just a voltage divider with a trimmer and 3A was all I could get it to go to.  3A actually required I power it from 15V which kinda upset the PNP driver for the fan.  It was merely toasty.

Next plan is to implement the better control circuitry with the opamp and voltage reference.

Other things I learnt today. 

1. Using overrated cables (12AWG) makes soldering connections difficult.  My 25 watt Weller struggled and I have several burns on my finger tips.  14 or even 16AWG probably would have done!

2. Soldering single core wire to mosfet pins is a nightmare.  They don't flex when the solder takes so any movement of your hand at all when you release the heat and the joint is bad.  Later I found it is better to strip back much more single core and wind it around the pin first, then just fill the winds with solder.  I might redo the gate joint this way.

Busy day:


The circuit:


The high current side:

* I'm using the CPU cooler's spring bracket and a high strength carbon fibre strut bar (from an RC heli) to hold the mosfet in place.  It is putting sizable (enough) force on the mosfet.

The Fan:

« Last Edit: December 03, 2017, 03:22:55 am by paulca »
"What could possibly go wrong?"
Current Open Projects:  3 Channel Audio mixer with DAC, BT, pre-amps and h/phone amp, WS281x LED controller Version 2 5V/5A w/Atmega328, FY6600 Power supply, 5A DC Load (Still!)
 

Offline paulca

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Re: DC Load
« Reply #32 on: December 03, 2017, 03:45:32 am »
Now I got to torture my bench supply charging that big LiPo.  The Tenma 30V 5A single version.  It's fan is very noisy when running at 5A but it worked for the 45 minutes or so it took to get the LiPo up to 12.6V and go to constant voltage.

Safety first, I had a cell balancer attached so I could see the individual cell voltages and let it attempt to balance them at the same time.

I had to pause the charge a few times to allow the cell balancer to catch up. Once one cell got to 4.21V I killed the charge current and waited on the balancer to do it's thing, then restarted the charge.

The cell balance is interesting to watch.  With:

4.20V  4.15V 4.17V reading,

It would switch into balance mode using the the higher cell to charge the lower cells and read:

4.17V  4.16V 4.17V for about 30 seconds, then it would pause and sense the cells for a while.

4.20V  4.16V 4.17V reading,

Back and forward between cross charging and sensing.  Until finally it read:

4.18 4.18 4.18 the "BALANCE END" buzzer went off and I restarted the charge current.

EDIT:
Now the downside.  While the power supply can charge the pack and the analyser can balance it, the balancer struggles to keep up, even when the PSU is back to 0.8A.  So I have to keep pausing the charge and I suspect as I get closer to full charge it will be more and more often.

The Accucel will come out to finalize the balance charge.  It has smarts in it which will not only lower the charge current based on how out of balance the cells are, but it can also actively charge individual cells (I believe).
« Last Edit: December 03, 2017, 03:51:23 am by paulca »
"What could possibly go wrong?"
Current Open Projects:  3 Channel Audio mixer with DAC, BT, pre-amps and h/phone amp, WS281x LED controller Version 2 5V/5A w/Atmega328, FY6600 Power supply, 5A DC Load (Still!)
 

Offline paulca

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Re: DC Load
« Reply #33 on: December 03, 2017, 11:10:41 pm »
Got the full Peter Oakes design version running on the breadboard this morning.

5A sustained resulted in a heatsink temp of 48*C stable (kitchen probe) and a max measured surface temperature on the mosfet of 61.3*C  (using an IR thermometer).  This was after 5 minutes sustained.

I had/have an issue with the trimmer on the Vref.  My trim pot maxs out and I can still only get 4.985A (though I'm not really complaining), I would have expected the trim on the Vref to allow adjustments below and above the ref. 

I also 'fixed' my fan controller.  Took out a load of motor noise with a 100uF cap across the motor feed wires and took out a load of bouncing/ringing/osculation on the comparator at transition with a 1uF bypass cap.  I also swapped round from using a PNP and sinking to the comparator output and used a pull up and an NPN as most of the datasheet applications choose this approach.  I could run it in hysteresis but I would need to carefully adjust my thermistor voltage divider and reference divider which I'm not sure I can be bothered for a fan controller.   The downside is it no longer soft starts and no longer winds up and down at marginal temps, it's now a hard cut on/off.  So it pulses every few seconds for a minute or so when cooling after a hot run.  Not sure which I prefer.  The bouncing was effectively adding PCM control to give it a softer switch on at lower temperatures.

Next.

Some form of metering for voltage and current.  I know this might destabilise the sense side, but I'm not really aiming for that great a precision.  Knowing the current and voltage of the load without having to use a DMM is more useful to me.  The downside is the only ammeter I have claims to be 75mV per 50A.   I think I might start a new thread to ask about this as it seems unreasonably low.
"What could possibly go wrong?"
Current Open Projects:  3 Channel Audio mixer with DAC, BT, pre-amps and h/phone amp, WS281x LED controller Version 2 5V/5A w/Atmega328, FY6600 Power supply, 5A DC Load (Still!)
 

Offline paulca

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Re: DC Load
« Reply #34 on: December 04, 2017, 08:25:25 am »
Heat.  Did some sums.  First figuring out what the sums where.  Then trying to find values for them.

I couldn't find a thermal resistance for my AMD cooler, but given the "Thermal Design Power" of the chip it is meant to cool is 68W, I should be okay at 5A 12V.

I also determined that even if I bought a £100 PC water cooler with dual 12" radiator (thermal resistance of circa 0.1 C/W) I could still only push it to 120W or 10A 12V.  Higher and higher priced units shaved another amp tops, but started to cost £200.  I can also get a little bit more by going to the 100A Mosfet as it seems to have better junction to case and case to sink TR, but the downside is, it has a lower max Jt.

So, staying with the basic CPU coolers, if I want more amps I will have to go parallel mosfets each with a CPU heatsink.  A task for way, way down the line.  5A is grand for what I need right now.

The 5 gallon heat sink bath of vegetable oil just sounds like a smelly mess waiting to happen.
"What could possibly go wrong?"
Current Open Projects:  3 Channel Audio mixer with DAC, BT, pre-amps and h/phone amp, WS281x LED controller Version 2 5V/5A w/Atmega328, FY6600 Power supply, 5A DC Load (Still!)
 


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